In the manufacture of dynamoelectric machines having laminated stator cores, it is desirable to retain the laminations in their stacked position under a pre-determined optimum compressive pressure. Such optimum pressure must be sufficiently great to prevent vibration of the laminations during operation of the machine, because such vibration could cause metal fatigue and eventual failure, as well as resulting in undesirable noise and possible chafing of the insulation on the energizing coils wound in the stator slots. On the other hand, the compressive pressure should not be such that excessive variations occur in core length and produce loss of needed magnetic material. Obtaining an optimum degree of compression of a laminated stator core is complicated by the fact that the stator laminations frequently vary slightly in thickness and the varnish coatings used to insulate the laminations frequently are of relatively uneven thickness. Normally, such variations in thickness on the individual laminations and in the varnish coating are relatively small but the cumulative effect of these variations in a complete stack of laminations needed for large dynamoelectric machine stators can cause a significant variation in stator lengths if appropriate control of the compressive pressures applied to the stator during manufacture is not maintained.
A wide variety of manufacturing methods and associated stator laminations clamping assemblies have been developed over the years. In general, such prior art methods and structures can be divided into two broad categories, i.e., those used to manufacture relatively large dynamoelectric machines that require several tons of pressure to be applied in obtaining a desired compressive force on the stacked stator laminations, and those used to assemble smaller dynamoelectric machines that require a much lower compressive force to be applied in assembling the stacked laminations. Generally speaking, in the manufacture of the larger type of dynamoelectric machines it is common practice to secure the stator laminations between a pair of clamping flanges positioned at opposite ends of the stacked laminations. At the present time, some variations of one of three well-known stator assembling methods is almost always used to retain a desired compressive force on the laminations of such large stator assemblies. In perhaps the most widely used of these known prior art methods, a stator-supporting frame is machined over its entire length so that it engages the periphery of each of the stator laminations at several circumferentially-spaced points. Then, the frame is heated to expand it sufficiently to receive therein the compressed stator laminations. As the frame cools, it shrinks around the laminations and secures them tightly in their compressed position. A second commonly used stator core clamping method employs a so-called "clam shell" clamping structure. Basically, the "clam shell" type of clamping assembly utilizes a plurality of threaded bolts positioned at arcuately spaced-apart points around the circumference of stator lamination clamping flanges to enable the flanges to be forced toward one another as the bolts are tightened. An early example of one form of such a stator clamping structure is shown in U.S. Pat. No. 1,685,054-Hibbard which issued on Sept. 18, 1928. The third stator core lamination clamping means now in common use typically incorporates one or more wedges or keys mounted between the stator clamping flanges the frame of a dynamoelectric machine to enable the compressive force applied through the flanges to the stacked laminations to be adjusted by relative movement of the wedges causing them to apply more or less force to the flanges, until a desired compressive force is attained.
All of the foregoing presently known prior art methods and structures for securing the stator laminations of large dynamoelectric machines under a desired pre-determined compressive force have certain features in common. In each of them, for example, lamination clamping means are used which apply a relatively static clamping force to the stack of laminations, rather than utilizing resiliently pre-stressed clamping means to obtain such a clamping force. The use of such static clamping methods results in a second characteristic feature inherent in each of the above-described prior art stator assembly structures. Specifically, each of these structures must be relatively massive to accommodate the high compressive forces that must be applied to a stack of laminations during initial assembly of the machine in order to allow the static-type clamping means to retain a desirable level of compressive force on the laminations after they have been released from a positioning press and are then held in position only by the clamping means. It has long been recognized that such massive structures have certain disadvantages, such as their inherent cost and the inconvenience and expense encountered in transporting them. However, prior to the present invention, these prior art methods appeared to be the most suitable for commercial manufacture of dynamoelectric machines.
In addition to the general types of stator lamination clamping means described above for use in the manufacture of stators for large dynamoelectric machines, several types of resilient stator lamination mounting means are known for application in the manufacture of smaller machines. Normally, such resiliently pre-stressed stator lamination clamping means are used primarily as an efficient means for quickly securing a stack of laminations in a desired position on a shaft, rather than being designed primarily to apply any appreciable compressive force to the stacked laminations. Examples of such low pressure securing or locking means for securing small stator laminations on a shaft are shown in U.S. Pat. No. 1,192,404-Ewart, which issued on July 25, 1916 and in U.S. Pat. No. 1,467,938-Janette, which issued on Sept. 11, 1923. Because the assembly methods and structures shown in these two patents are not capable of applying a high compressive pressure to the stacked laminations, they are not suitable for applying a compressive pressure of several tons that is needed in the manufacture of stator lamination assemblies for larger machines.
Another type of stator clamping means that is suitable for small and medium size machines is shown in U.S. Pat. No. 2,876,371-Wesolowski, which issued on Mar. 3, 1959 and is assigned to the assignee of the present invention. A form of resilient clamping means are employed in the Wesolowski arrangement to secure lamination clamping rings under a desired pre-determined clamping pressure. However, the resilient clamping means used comprise a plurality of pins, the respective ends of which are welded to the lamination clamping rings when the stack of laminations is held under a compressive force. The length of the pins is pre-determined so that when the compressive force on the stack of laminations is released, the pins are stretched beyond their elastic limit by the expansion of the laminations. The stated purpose of such stretching of the pins is to stabilize the compressive force that they apply to the clamping rings. In certain respects, the Wesolowski clamping arrangement is similar to the relatively static type of clamping means described above in that the ring-securing pin must be sufficiently massive, or a large number of pins must be used, so that the high compressive force needed to yield a desired retained force of compression after the pins are stretched is afforded. Moreover, the structure and method disclosed in the Wesolowski patent is not suitable for use with very large stator core assemblies in which several tons of compressive pressure is needed to attain an optimum balance between core flux carrying capacity and core flux losses.
Accordingly, it is a primary object of the present invention to provide a dynamoelectric machine core assembly and method for making such an assembly, which overcome the disadvantages inherent in known prior art methods and structures for compressively clamping laminated core structures for large dynamoelectric machines.
Another object of the invention is to provide a laminated core clamping assembly and method of manufacture that affords an optimum pre-determined retained compressive force in a stack of laminations following the final assembly of such a stack.
A further object of the invention is to provide a dynamoelectric machine core assembly for large dynamoelectric machines, that are less costly to manufacture and lighter in assembled weight than prior art machines having equivalent retained compressive force in their cores.
Still another object of the invention is to provide a method of manufacturing a laminated core assembly that enables a desirably high pre-determined compressive force to be maintained in the finished assembly while permitting the use of relatively low lamination compressing forces during the manufacturing processes.
Yet another object of the invention is to provide a dynamoelectric machine core assembly having an optimum retained compressive force in its laminated core stack while utilizing relatively small and lightweight structural assemblies for maintaining the core pressure on the laminated core stack, after it is removed from a core-compressing press.
Additional objects and advantages of the invention will become apparent to those skilled in the art from the description of it that follows below, taken in conjunction with the illustrations attached hereto.